It is often necessary, or at least desirable, to concentrate a liquid mixture by removing a portion of the solvent, generally water, from the liquid mixture. The resulting product, therefore, is in a more concentrated form. It has been common to concentrate radioactive waste (rad waste), cooling tower blow down waste water, fruit and vegetable juices such as orange juice, grapefruit juice, grape juice, and tomato juice by evaporation to remove water. In addition, seawater and brackish water have been concentrated by evaporation, although the condensed vapor has been recovered as usable potable water rather than discarded as in concentrating fruit and vegetable juices. Nevertheless, each is a concentrating process. In the case of juice, the concentrate is the desirable product; whereas, in obtaining potable water from seawater or brackish water the concentrate is discarded.
Evaporative concentration as described, as well as evaporation of chemical solutions or liquid dispersions, requires substantial energy since it relies on the latent heat of vaporization. Scaling of equipment and enhanced corrosion are often inherent at the temperatures involved in evaporative concentration. Loss of flavor and aroma also result during evaporative concentration of food products.
Because of the shortcomings involved in evaporative concentration, it has been found advantageous to freeze concentrate many products, particularly those having water as the liquid carrier. Generally, reduced energy is required since freeze concentrating relies on the heat of fusion instead of the heat of evaporation. In such a process, water is removed by first producing ice crystals which are then separated from the concentrate. Next, the ice crystals are washed to remove the remaining concentrate on them. The ice crystals can then be discarded or melted if potable water is desired.
Engdahl U.S. Pat. No. 4,314,455 discloses a freeze concentration apparatus and process. In FIG. 2 of that patent a two stage freeze concentration system is disclosed. The system employs a second freeze exchanger to cool liquid in a second or warm concentrator vessel thus increasing capital cost. In addition, the aqueous mixture which is separated from the ice formed in the process is recirculated to the warm concentrator from which it is subsequently pumped to the primary or first freeze exchanger. This approach utilizes a pump and extra energy which desirably should be avoided if possible.